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packages/reactivity-core/README.md

@@ -101,6 +101,7 @@ Computed signals can be watched (e.g. via `effect()`) like any other signal:
 
 ```ts
 import { reactive, computed, effect } from "@conterra/reactivity-core";
+
 const age = reactive(21);
 const doubleAge = computed(() => age.value * 2);
 
@@ -113,8 +114,134 @@ effect(() => {
 age.value = 22;
 ```
 
-Note that the callback function used by a computed signal should be stateless:
-it is supposed to compute a value (possibly very often), and it should not change the state of the system while doing so.
+Computed signals only re-compute their value (by invoking the callback function) when any of their dependencies have changed.
+For as long as nothing has changed, the current value will be cached.
+This can make even complex computed signals very efficient.
+
+Note that the callback function for a computed signal should be stateless:
+it is supposed to compute a value (possibly very often), and it should not change the state of the application while doing so.
+
+### Using signals for reactive object properties
+
+You can use signals in your classes (or single objects) to implement reactive objects.
+For example:
+
+```ts
+import { reactive } from "@conterra/reactivity-core";
+
+class Person {
+    // Could be private or public
+    _name = reactive("");
+
+    get name() {
+        return this._name.value;
+    }
+
+    set name(value) {
+        // Reactive write -> watches that used the `name` getter are notified.
+        // We could use this setter (which could also be a normal method) to enforce preconditions.
+        this._name.value = value;
+    }
+}
+```
+
+Instances of person are now reactive, since their state is actually stored in signals:
+
+```ts
+import { effect } from "@conterra/reactivity-core";
+
+// Person from previous example
+const p = new Person();
+p.name = "E. Example";
+
+// Prints "E. Example"
+effect(() => {
+    console.log(p.name);
+});
+
+// Triggers effect again; prints "T. Test"
+p.name = "T. Test";
+```
+
+You can also provide computed values or accessor methods in your class:
+
+```ts
+import { reactive, computed } from "@conterra/reactivity-core";
+
+// In this example, first name and last name can only be written to.
+// Only the combined full name is available to users of the class.
+class Person {
+    _firstName = reactive("");
+    _lastName = reactive("");
+    _fullName = computed(() => `${this._firstName.value} ${this._lastName.value}`);
+
+    setFirstName(name) {
+        this._firstName.value = name;
+    }
+
+    setLastName(name) {
+        this._lastName_.value = name;
+    }
+
+    getFullName() {
+        return this._fullName.value;
+    }
+}
+```
+
+### Effect vs. Watch
+
+We provide two different APIs to run code when reactive values change.
+The simpler one is effect `effect()`:
+
+```js
+import { reactive, effect } from "@conterra/reactivity-core";
+
+const r1 = reactive(0);
+const r2 = reactive(1);
+const r3 = reactive(2);
+
+// Will run whenever _any_ of the given signals changed,
+// even if the sum turns out to be the same.
+effect(() => {
+    const sum = r1.value + r2.value + r3.value;
+    console.log(sum);
+});
+```
+
+If your effect callbacks become more complex, it may be difficult to control which signals are ultimately used.
+This can result in your effect running too often, because you're really only interested in _some_ changes and not all of them.
+
+In that case, you can use `watch()`:
+
+```js
+import { reactive, watch } from "@conterra/reactivity-core";
+
+const r1 = reactive(0);
+const r2 = reactive(1);
+const r3 = reactive(2);
+
+watch(
+    // (1)
+    () => {
+        const sum = r1.value + r2.value + r3.value;
+        return [sum];
+    },
+    // (2)
+    ([sum]) => {
+        console.log(sum);
+    }
+);
+```
+
+`watch()` takes two functions:
+
+-   **(1)**: The _selector_ function. This function's body is tracked (like in `effect()`) and all its reactive dependencies are recorded. The function must return an array of values.
+-   **(2)**: The _callback_ function. This function is called whenever the selector function returned different values. The callback itself is _not_ reactive.
+
+In this example, the callback function will only re-run when the computed sum truly changed.
+
+### Reactive collections
 
 ## API
 
@@ -124,6 +251,8 @@ it is supposed to compute a value (possibly very often), and it should not chang
 
 ### Collections
 
+### Types
+
 ## Gotchas
 
 ### Avoid side effects in computed signals